Stellar flares detected with MAXI for the past 7 years
Yohko Tsuboi1 , Ryo Sasaki1 , Yumiko Nakamura1 , Ken Yabuki1 , Atsushi Kawagoe1 ,Soichiro Kaneto1 , Yasuharu Sugawara2 , Hitoshi Negoro3 , and the MAXI Team1

Department of Physics, Faculty of Science and Engineering, Chuo University,
1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan
2
ISAS, JAXA, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 229-8510, Japan3
Department of Physics, Nihon University, 1-8 Kanda-Surugadai, Chiyoda-ku, Tokyo 101-8308, JapanE-mail(YT): tsuboi@phys.chuo-u.ac.jp
Abstract
Since the launch in 2009 August, with the unprecedentedly high sensitivity as an all-sky X-ray monitor,
MAXI has caught more than a hundred of huge flares from stars. Most of them are from low-mass, activestars (RS CVn systems, an Algol system, dMe systems, a dKe system, Young Stellar Objects). With thetotal radiative energy of 1034−39 ergs, the MAXI detections have broken the record of the largest flaring
magnitudes in each stellar categories (e.g. “RS CVn” and so on). The enlarged sample of intense flares
has enabled us to do systematic studies in various viewpoints. One of the studies is our discovery of auniversal correlation between the flare duration and the intrinsic X-ray luminosity, which holds for 5 and
12 orders of magnitude in the duration and LX , respectively (Tsuboi et al. 2016).
Besides low-mass stars, a historically brightest X-ray flare has been detected with MAXI from themassive star system, Eta Carinae (Negoro et al. 2014).
In this review, we introduce the studies of stellar flares obtained in 7.25-year monitoring with MAXI.
Key words: stars: activity — stars: flare — stars: late-type — stars: rotation — stars: variables:
general1.

Introduction

Stellar flares are thought to be a resultant of magnetic reconnection on a stellar surface. The closest stellar flares
to us are solar flares. The typical total energy of a solar flare ranges from 1029 to 1032 erg (e.g., Shibata &Yokoyama 2002), and the duration is several minutes
to several hours (e.g., Shimizu 1995). Despite the long
history of the observation of the solar flares, the highenergy cut-oﬀ of the solar flares in the occurrence rate(dN/dE) versus flare energy (E) has not yet been reported. Whereas, in 2012, Maehara and his co-workers
discovered 365 flares with the total energy of 1033 –1036
erg, in the range of so called “super-flare” (Schaefer et al.
2000), on 148 G-type dwarfs which includes 10 Sun-likestars (Maehara et al. 2012). This frightens us, because
our civilization would suﬀer from much more severe damages, if superflares were really to occur on the Sun.
Even if we are apart from the solar flares, generally,
we cannot answer the following fundamental questions;
how large flares can a star have, and how is such a flare?
The best way to answer such questions would be to use
MAXI, which has the unprecedentedly high sensitivity

for an all-sky X-ray monitor. Here, we report the resultswith the gas proportional counters (GSC) of MAXI obtained in the first 7.25-year operation from 2009 August
to 2016 November.
2. Results
2.1. Flare sample
We detected 106 flares on twenty-seven low-mass stars;
fourteen RS CVn systems, one Algol system (Algol),nine dMe stars, one dKe star, one Young Stellar Object(TWA-7) and one K-type variable star. The detectionof the flare from TWA-7 and the results from the first
two-year monitoring with MAXI have been already reported in Uzawa et al. (2011) and Tsuboi et al. (2016),
respectively. Most of the categories are classified as active binaries, which consist of the pair of a sub-giant
and a main sequence, and a pair of a giants and mainsequence, and a pair of two main-sequence stars. Thedistance between each stellar component are 1–2 timesof their own stellar sizes. We should note that our sample contains single active stars, although they are minor(Uzawa et al. 2011 and Tsuboi et al. 2016).

RS CVndMe stars

Rsolar
10 mCrab

Loop length

Magnetic Field

Fig. 1. Log-log plot of X-ray luminosity in the 2–20 keV band of flaresvs. distance from stars detected with MAXI/GSC. The squares,triangles, circle, diamond, and star show RS-CVn type stars, dMestars, dKe star, Algol, and TWA-7, respectively. The detection
limit appears to be roughly 10 mCrab in the 2–20 keV band.

Figure 1 shows the X-ray luminosity vs. distance tothe source. This shows that the MAXI sources are within
about 200 pc, and our detection limit is roughly 10
mCrab. The largest flares that we detected in seven-year
monitoring have the luminosity of 1034 ergs s−1 . Thanks
to the improvement of the alert system “nova search”(Negoro et al. 2016) during 7.25 years, all dMe stars
within 10 pc distance have been detected with MAXI.
On the other hand, the detected sample of active binaries are only the tip of the iceberg; within 100 pc distance, 256 active binaries are known to exist, but only
1/10 are detected. Moreover, none of the solar type stars
has been detected, despite that within 20 pc distances,
there are 15 such stars.
The total energies of the detected flares range from1034 to 1039 erg in the 2-20 keV band, i.e. at least, morethan two orders of magnitude larger than the maximum
solar flare (1032 erg). The detail is reported in Sasaki etal. (2017).
2.2. Universal correlations
Figure 2 shows the plot of emission measure vs. plasmatemperature. It had been known that there is a universal
correlation which holds through many orders of magnitudes in emission measure (Shibata & Yokoyama 1999).
MAXI sources lie on the high-end of the correlation. The
large emission measures in the MAXI sources indicatethat the radiative volume, and then the loop size, arelarger than those in the solar flares by many orders ofmagnitudes, and the magnetic field, which is indicativeof the plasma temperature, is on the other hand not sodiﬀerent from those in solar flares. In some flares, theFig. 2. Log-log plot of emission measure vs. plasma temperature(kT ) for the MAXI X-ray flares (filled symbols), along with stellar
flares from RS-CVn type, Algol, dMe stars and YSOs, solar flares,and solar microflares. The arrows indicate the lower limits forindividual MAXI/GSC sources. EM -kT relations, for B= 15, 50,and 150 Gauss and for the loop-sizes of 108 , 1010 , 1012 , and 1014
cm, from Shibata & Yokoyama (1999) are also indicated.

loop size should be more than 10 times larger than eventhe distance between each binary component.
Recently, Tsuboi et al. (2016) discovered another universal correlation between the intrinsic X-ray luminosity
in the 0.1–100 keV band and the duration of flares. Figure 3 shows the correlation which holds from solar micro
flares to the large flares detected with MAXI/GSC. The
MAXI flares are on the high-end of the positive correlation.
2.3. Serendipitous flares from HD347929
We have detected the flares from HD347929 in 2013
serendipitously and also have recognized that the source
had another flare in 2010. Since we had possessed almost
no information of this source, we conducted a spectroscopic observation in the optical band with Gumma observatory, and obtained a hint for the nature; it seems tobe a young stellar object with a rich Li abundance. The
detail will be reported elsewhere (Tsuboi et al. 2017).
2.4. The historically brightest X-ray flare on the massive
star system, Eta Carinae
MAXI detected not only the flares on the low-mass stars
but also those on a massive stellar system, Eta Carinae.
Eta Carinae is a mystery source with surrounding dust
nebulae. The dust erupted from Eta Carinae itself inthe 19th century. There is also an X-ray emitting nebula, surrounding Eta Carinae, which is located outside ofthe dust nebulae. Then the nature of the central source
has been little known, except for the information thatit seems to be a stellar system containing at least two

Duration ~ Lx0.2

SunStars
Stars(pointing) (MAXI)

Fig. 3. Log-log plot of duration of flares vs. X-ray luminosity in the 0.1–100 keV band. The best-fit model is inserted with a broad solid line
(Tsuboi et al. 2016).

massive stars. This is indicated from the periodic X-rayvariation with 5.54 year period. Such variation is widely
observed in wind colliding binaries, with the ellipticalorbit and the enormous stellar winds from both stellar
components. The X-ray intensities become maximum at
the periastron.
At the last periastron of Eta Carinae in 2014 June,MAXI caught the highest X-ray flare among those everrecorded (Negoro et al. 2014). The long X-ray monitoring with MAXI toward the next periastron is highly
deserved.
3.

Summary• 107 flares from 27 low-mass stars were detected.
• dMe stars within 10 pc were all detected.
• No flare has been detected from G type main sequence stars within 20 pc, so far.
• A universal correlation was obtained in the durationof flares vs. the bolometric X-ray luminosity.
• Serendipitously, huge flares were detected from one
un-classified stellar source.
• The highest X-ray flare ever recorded was detectedfrom Eta Carinae.

References
Maehara, H., Shibayama, T., Notsu, S., et al. 2012,
Nature, 485, 478
Negoro, H., Fukushima, K., Mihara, T., Takagi, T., etal. 2014, ATEL#6289
Negoro, H., Kohama, M., Suzuki, M., et al. 2016, PASJ,68, S1

Sasaki, R., Tsuboi, Y., Katsuda, T., et al. 2017, thisproceedings
Schaefer, B. E., King, J. R., & Deliyannis, C. P. 2000,ApJ, 529, 1026
Shibata, K., & Yokoyama, T. 1999, ApJL, 526, L49
Shibata, K., & Yokoyama, T. 2002, ApJ, 577, 422
Shimizu, T. 1995, PASJ, 47, 251
Tsuboi, Y., Yamazaki, K., Sugawara, Y. 2016, PASJ, 68,
90
Tsuboi, Y., Kawagoe, A., Sugawara, Y. 2017, inpreparation
Uzawa, A., Tsuboi, Y., Morii, M., et al. 2011, PASJ, 63,713